Water flooding as an oil recovery technique has been in use since 1890 when operators in the US realised that water entering the productive reservoir formation was stimulating production. In some cases, water is supplied from an adjacent connected aquifer to push the oil towards the producing wells. In situations where there is no aquifer support, water is typically pumped into the reservoir through dedicated injection wells. The water phase replaces the oil and gas in the reservoir and thereby serves to maintain pressure. Recovery factors from water flooding vary from 1-2% in heavy oil reservoirs up to 50% in light oil reservoirs with typically values around 30-35%, much lower than the microscopic sweep efficiency of 70-80%.
A reason for sub-optimal recovery factors is related to the macroscopic sweep, which in turn is a reflection of reservoir heterogeneity and fluid mobility ratios. Fluid mobility ratio may be controlled to some extent by adding viscosifying agents to the injection phase, such as polymers or foams, but the presence of large permeability variations requires a different approach to improve macroscopic sweep. An extreme case is a direct high-permeability conduit, either natural or induced, between an injector and one or more producers, which requires complete or at least partial plugging of the high-permeability conduit. This process is known as conformance control.
Conformance treatments can significantly improve the sweep efficiency of a malfunctioning water flood and is a prerequisite for any Enhanced Oil Recovery (EOR) method. Conformance control generally requires a combination of mechanical and chemical solutions. The role of the mechanical part is to ensure that the chemicals reach the part of the reservoir, which they are intended to plug. Although commercial chemicals already exist for plugging high-permeability zones, the chemical mixture has to be tailored to a particular application, depending on salinity, temperature, pore size etc. When two or more chemicals are required to react and plug a high-permeability zone, the reaction may also cause plugging of other regions of the formation, such as low-permeability zones, thereby lowering productivity during subsequent oil recovery.
Attempts have been made to reduce the permeability of selected zones during profile control.
U.S. Pat. No. 4,848,464 and (Jennings et al.) disclose a method comprising injecting a solidifiable gel containing a gel breaker into a formation where it enters a zone of lesser and a zone of greater permeability. Said gel blocks pores in the zone of lesser permeability. Another solidifiable gel lacking a gel breaker is then injected into the zone of greater permeability where it subsequently solidifies. The gel contained in the zone of lesser permeability (containing a gel breaker) liquefies, thereby unblocking this zone. Afterwards, a water-flooding enhanced oil recovery method is directed into the zone of lesser permeability.
It is amongst the objects of the present invention to obviate and/or mitigate at least one of the aforementioned disadvantages.